Erbium-doped GaN crystals as solid-state high energy laser gain medium



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Erbium doped GaN (Er:GaN) is a promising candidate as a novel gain medium for solid-state high energy lasers (HELs) due to its superior physical properties over synthetic garnet such as Nd:YAG. Er:GaN emits in the 1.5 m region, which is retina-safe and has a high transmission in the air. GaN/Er:GaN/GaN core-cladding planar waveguides (PWGs) were synthesized by metal-organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE); and their optical properties were studied and discussed. Optical confinement factor as functions of layer thickness and Er concentration of the Er:GaN core have been studied to guide the material growth and fabrication of these core-cladding PWGs. It was found that for an Er doping concentration of 3 x 1019 cm-3 in the core, the measured optical loss coefficients at 1.54 µm were 1.0 cm-1 for TE and 1.2 cm-1 for TM polarization modes, respectively. The results provided insights into approaches to further reduce the optical loss as well as an optimal configuration to resonantly pump GaN/Er:GaN/GaN PWGs by photons with λpump = 980 nm for achieving amplification and lasing at 1.54 µm. Photoluminescence (PL) studies were performed on Er:GaN epilayers synthesized by HVPE technique. The room temperature PL spectra of HVPE grown Er:GaN epilayers resolved as many as 11 and 7 emission lines in the 1.5 and 1.0 µm wavelength regions, respectively, corresponding to the intra-4f shell transitions between Stark levels from the first (4I13/2) and the second (4I11/2) excited states to the ground state (4I15/2) of Er3+ in GaN. The observed peak positions of these transitions enabled the construction of the detailed energy levels in Er:GaN. The results agree well with those of calculation based on a crystal field analysis. Precise determination of the detailed energy levels of the Stark levels in the 4I11/2, 4I13/2, and 4I15/5 states are critically important for the realization of HELs based on Er:GaN. Other than the preferred configuration of Er3+ occupying the Ga site with 4 nearest neighbor N atoms (ErGa), Er can also form a complex with a defect (ErGa-DX) in Er:GaN. A set of Er:GaN semi-bulk crystals were grown by HVPE at different growth temperatures to allow the determination of the formation energies, EF, of Er optical centers in Er:GaN. PL emission spectra near 1.5 µm pumped by a near band-edge excitation (λexc = 375 nm) and by a resonant excitation (λexc = 980 nm) were measured, which yielded two different formation energies of EF = 2.8 eV and 3.3 eV, corresponding to the formation of ErGa-DX and ErGa in Er:GaN, respectively. As a gain medium, the formation of ErGa-DX in Er:GaN would not only reduce the pumping efficiency of Er optical centers, but also increase the optical loss. Our results provide useful insights into possible strategies for enhancing the fraction of ErGa in Er:GaN and hence the pumping efficiency, paving the way for achieving optical gain and lasing in Er:GaN. Effects of polarization field on the Er3+ intra-4f shell transitions in GaN crystal host have been investigated via comparison PL emission spectroscopy studies conducted on erbium doped GaN (Er:GaN) and YAG (Er:YAG) crystals. The dominant optical transitions between the Stark levels of the first excited state (4I13/2) and ground state (4I15/2) in Er:GaN and Er:YAG were compared and analyzed, from which the Stark energy level diagrams have been constructed. It was observed that the dominant optical transitions in Er:GaN are between the Stark levels of the same irreducible representations; whereas those in Er:YAG are between the Stark levels of different irreducible representations. The unique selection rules in Er:GaN is a consequence of the presence of a net local polarization field acting on Er due to the wurtzite crystal structure of GaN. The results obtained here provide useful insights into understanding of the dominant optical transitions as well as the most probable emission lines to be utilized to achieve lasing in Er:GaN.

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High Energy Lasers, Laser Gain Medium